Dynamic range compressor image amplifier

ABSTRACT

A photoconductor-electroluminescent light-responsive device capable of responding to incident input light illumination images over a wide range of image intensities without distorting or blurring the image being produced thereby on the output.

United States Patent Inventor James E. Dueker St. Louis County, Mo.

Appl. No. 806,018

Filed Mar. 4, 1969 Patented Oct. 5, 1971 Assignee McDonnell DouglasCorporation St. Louis, Mo.

Continuation-impart of application Ser. No. 573,657, Aug. 19, 1966, nowabandoned.

DYNAMIC RANGE COMPRESSOR IMAGE AMPLIFIER 10 Claims, 5 Drawing Figs.

U.S. C1 250/213,

3 13/ 108 Int. Cl H011 17/00 Field of Search 250/2 I 3,

[56] References Cited UNITED STATES PATENTS 2,989,643 6/1961 Scanlon 1250/230 3,054,900 9/1962 Orthuber.. 250/213 3,187,184 6/1965Tomlinson... 1. 250/213 OTHER REFERENCES Tomlinson. Journal of theBritish Institute of Radio Engineers; Volv 17;No. 3;March 1957 pp.141-148 I-Iausmann-Slack: Physics Van Nostrand Co. Inc.; 4th Ed.,Copyright 1957; page 418 Primary Examiner-Walter StolweinAuorney-Charles B. Haverstock ABSTRACT: Aphotoconductor-electroluminescent lightresponsive device capable ofresponding to incident input light illumination images over a wide rangeof image intensities without distorting or blurring the image beingproduced thereby on the output.

DYNAMIC RANGE COMPRESSOR IMAGE AMPLIFIER This is a continuation-in-partof my earlier filed copending application Ser. No. 573,657, filed Aug.19, l966 now abandoned, and assigned to the same assignee.

The present invention relates generally to electro-optical devices anddevice particularly to light-sensitive devices including night sightingdevices capable of operating effectively over a wide range of lightconditions.

Attempts have been made in the past to devise night sighting means forline-of-sight operation, particularly for use under minimal lightconditions such as under starlight conditions. There is need for such adevice particularly as a means for reliably and accurately aiming fieldweapons and the like under almost total darkness conditions. All knownand available night sighting devices sufficiently sensitive for use withstarlight illumination cannot at the same time accommodate much of arange in brightness from one point to another in the field of view. Onthe other hand, all known night sighting devices that can accommodate alarge range in brightness from one point to another in the field ofview, are also relatively insensitive. Furthermore, all known devices,particularly of this second class require, among other things, arelatively intense source of light, or they require their own source oflight or other energy. For these and other reasons all known andavailable night sighting devices are unsatisfactory. The known devicesare also relatively expensive and are too complicated and cumbersome forfield use, they have relatively limited scope and range, the intensityand other characteristics of their light source are relativity critical,and they have the further disadvantage that the energy or light raysthey employ can also be used as a target by an enemy. For these andother reasons, known night sighting devices have enjoyed relativelylimited use or have not been used at all.

The present invention teaches the construction and operation of noveldynamic range compressor means for use with night sights and otherdevices, which means enable such devices to overcome these and otherdisadvantages and shortcomings. The subject means are capable ofoperating over a wide range of light conditions including minimallighting conditions such as starlight reflected by remote objects ortargets as well as relatively intense light conditions. Furthermore, thesubject means are able to respond to such reflections and producevisible images of a field of view so that an operator using a nightsight constructed according to the present invention can distinguishremote objects and accurately sight on a particular target. When thesubject means are being used under such minimal light conditions, theymay, from time to time, be exposed to more intense light conditions. Atsuch times it is important that the operator not lose sight of thetarget due to blooming of the image on the light-sensitive means. Thisis possible using the subject means because of the dynamic rangecompressor characteristics which are built into it. The means foraccomplishing this include a specially constructed multilayerlight-sensitive element having dynamic light range compressioncharacteristics which prevent undesirable blooming of images due tochanges in the intensity of the incident light impinging thereon.

in one embodiment, the present invention comprises means for sighting ontargets and other objects under nighttime conditions, as for example,when the only continuous source of incident light is the starlightreflected by remote objects. This embodiment includes a multilayerlight-sensitive element and optical means for exposing said element tothe incident starlight reflections from a remote field of view, saidlight-sensitive clement including a photoconductive layer positioned tobe exposed to the incident reflection, a layer of optically opaquedielectric material adjacent to the photoconductive layer in theelement, a layer of electroluminescent material on the opposite side ofthe opaque layer, means for establishing a voltage between oppositesides of the photoconductive and electroluminescent layers, means foramplifying the responses produced by the element to produce visibleimages of the field of view and optical means for observing theamplified image responses produced on the electroluminescent layer.

A principal object of the present invention is to provide night sightingmeans capable of operating on minute amounts of light such as reflectedstarlight.

Another object is to provide a light-sensitive element constructed toresist image blooming when subjected to widely varying lightintensities.

Another object is to provide a night sight device which does not requireits own source of radiation energy.

Another object is to provide a line-of-sight,night sighting device whichhas a substantial range.

Another object is to provide a light-sensitive element capable ofmaintaining a substantially linear relationship between output imagebrightness as a function of input image brightness particularly atrelatively low light intensities.

Another object is to provide means to control blooming in solid-statelight-sensitive devices including devices having electroluminescent andphotoconductive materials.

Another object is to control image bloomingin light-sensitive devicesthrough the use of optical isolation layers and/or pattern deposition ofthe layers of such devices.

Another object is to control or prevent blooming in lightsensitivedevices such as light amplifiers by means which substantially restrictthe dynamic range of light levels to be amplified.

These and other objects and advantages of the present invention willbecome apparent after considering the following detailed specificationwhich covers a preferred embodiment of the subject device in conjunctionwith the accompanying drawings, wherein:

FIG. 1 is a schematic view of a night sighting device constructedaccording to the present invention;

FIG. 2 is an enlarged perspective view showing the details of aphotoconductive-electroluminescent solid-state dynamic range compressorfor use in the device of FIG. 1;

FIG. 3 is a graph of output brightness in lumens per square meterplotted against incident illumination in lumens per square meter;

FIG. 4 is a graph of response time in seconds as a function of incidentillumination in lumens per square meter; and,

FIG. 5 is a graph of emitted brightness in lumens per square meterplotted against image diameter in centimeters,

Referring to the drawings more particularly by reference numbers, number10 refers generally to a night sighting device constructed according tothe present invention. The device 10 includes a primary objective mirror12 which faces toward an observation field or target area to beobserved, and a secondary mirror 14 which receives incident image lightreflected by the mirror 12 and reflects it in the directions shown bythe arrows. The device 10 also includes a light-sensitive element 16which will be referred to as a dynamic range compressor because of itspeculiar operating characteristics. The structural details of theelement 16 are shown inFlG. 2.

The incident light impinging on the element 16 from the mirror 14produces an image of the observed field of view which can be amplifiedand viewed by an observer. The means by which the image produced on theelement 1.6 can be viewed include a transfer lens 18, an imageintensifier device 20, and an eyepiece 22. The optical means may alsoinclude crosshairs for aiming purposes if the device is to be used foraiming a weapon.

Of particular importance to the present invention is the constructionand operation of the semiconductor elements 16 which acts as a dynamicrange compressor. The term dynamic range compressor refers to theability of, the element 16 to respond to incident light of widelyvarying intensities without producing objectionable blooming of theimage which is a condition that causes the image to expand and becomeblurred and indistinct as the intensity of the incident light increases.This occurs because at the more intense incident light conditions thelight-sensitive element becomes overexcited. This objectionablecondition occurs in all known light amplifiers employing photocathodesand existing light-sensitive elements that include photoconductiveand/or electroluminescent layers. The present element constructionovercomes the objectionable blooming condition and at the same time isable to respond to an extremely wide range of light intensitiesincluding the minute amounts of light such as are produced by starlightafter it has been reflected from remote objects. Furthermore, thesubject element construction prevents an operator from losing sight ofan object due to momentary changes in the intensity of the incidentlight.

FIG. 2 shows the structural details of one embodiment of the lightsensitive range compressor element 16. The element 16 includes a glassor other transparent substrate layer 24 on which is applied a layer 26of a highly transparent material such as stannous oxide. The layer 26 isalso electrically conductive and is sometimes referred to as a Nesalayer. The layer 26 is used as one electrode of the element 16. Thereare other transparent conductive materials that can also be used for thelayer 26 and it is not intended to restrict the construction of thelayer 26 to a particular substance.

A photoconductive (PC) layer 28 is applied or attached to the layer 26and the opposite surface of the layer 28 from the layer 26 has a layerof optically opaque dielectric material 30 attached thereto, as shown.In like manner, the other surface of the dielectric layer 30 is attachedto a layer 32 of electroluminescent-phosphor (EL) material or the like.The opposite surface of the electroluminescent layer 32 has atransparent conductive layer 34 of a material such as gold appliedthereto, and the layers 26 and 34 are connected across a source ofelectric potential.

The element 16 is mounted in the position shown in FIG. 1 when used in asighting device with the transparent substrate or glass layer 24 on theside that faces the incident light from the field of view. The selectionof materials from which the layers of the element 16 are constructed isimportant to the operation and response characteristics of the device.There are many known materials which can be used for the differentlayers of the elements 16, and it is not intended in this specificationto limit or restrict the construction of any layer to a particularmaterial or substance because the operating characteristics will varydepending on the materials selected. This is especially so of the PC andEL layers.

The selection of the materials for the photoconductive layer 28 isparticularly important whether the device is to be used as an imageamplifier or for dynamic range compression. For example, the selectionof the photoconductive layer material in large measure determines theminimum light intensity or level to which the device will respond andthe speed of response as well as the spectral range over which thedevice will operate. The speed and sensitivity interdependence arebetter understood by considering the gain equation for a photoconductivematerial which is written:

G is the gain of a photoconductor defined as the ratio of the number ofcharge carriers in the photocurrent to the number of photons absorbed.

p. is the charge carrier mobility,

v is the voltage applied across the two electrodes separated y d thedistance, of separation.

L is the number of photons absorbed per unit volume per unit time,

A is proportional to the recombination rage of free electrons and holes,

B is proportional to eion where ion is the ionization energy of traps,

D is a constant related to the recombination of electrons with trappedholes, and

N is the number of trapping levels per unit volume.

The only parameters in the above equation which are independent of thephotoconductive material itself and hence which can be selected are theapplied voltage V and the distance of conduction d between the conductorlayers 26 and 34. All of the other terms are controlled by the selectionof the particular photoconductive material for the layer 28. It is to benoted, however, that the gain G cannot be increased without limit byincreasing the value of the electric field term V/d because ofpossibility of breakdown. Some compromise must therefore be reached.

The output brightness produced on the electroluminescent layer is afunction of the voltage applied thereacross and the brightness varies insuch a way that the relationship between the applied voltage and theoutput brightness is a direct proportional relationship. Also themaximum output brightness obtainable from the electroluminescent layersuch as the layer 32 is limited by the dielectric strength of the layer,the dielectric strength being the electric field intensity which willcause an electrical breakdown of the layer. In this regard, it isimportant to note that the electroluminescent layer in the presentdevice will never saturate but will breakdown if the electric fieldthereacross which depends on the applied voltage is made too strong sothat arcing and accompanying breakdown occurs resulting in destructionof the electroluminescent layer. If this occurs then theelectroluminescent layer as well as the entire device would becomeuseless.

The photoconducting layer 28 on the other hand, is constructed ofmaterials which are intrinsically photoconductors and as such involvecharge carrier transitions between their valence and their conductionbands. This type of photoconduction does not saturate with highintensity light, and in fact the population in the valence band of thephotoconductive layer is so high that it never saturates. Impurityphotoconduction on the other hand which occurs when the impurities inthe photoconducting layer produce available charge carriers at energylevels that are between the valence band and the conduction band willsaturate but usually not until about 5 to 7 decades of brightnessincrease has occurred. This fact is made use of in the present device.In other words, in the present device the photoconducting layer willsaturate when an impurity photoconduction layer is used but theelectroluminescent layer will never saturate if the voltage appliedthereacross is never permitted to become so large as to cause arcing.

The total present dynamic range compressor device which includes bothelectroluminescent and photoconducting layers saturates after 2 or 3decades of increase in the level of the input brightness. The saturationof the total device which occurs, r, is not a result of saturation ofthe photoconducting layer for the reasons stated above, nor is it aresult of saturation of the electroluminescent layer which neversaturates. In this regard it should be noted also that thephotoconductive layer employed in the present device is constructed of amaterial that is capable of responding to the input light intensity evenafter the input light intensity is sufficient to cause the total deviceto saturate, that is even after the input brightness has increased 2 or3 decades. How the saturation of the output occurs in the subject deviceis explained below, but at this point it is only important to recognizethat it is not produced by saturation of the electroluminescent layer,since as stated above the electroluminescent layer never saturatesunless of course it breaks down and becomes useless due to arcing causedby an excessive high voltage being applied thereacross.

Laboratory measurements made on devices constructed according to thesubject dynamic range compressor have demonstrated that thephotoconducting and the electroluminescent layers can be treatedeffectively as though they were high-leakage capacitors connected inseries each effectively having as variable impedance or resistanceconnected across it. The impedance of the variable impedance across thecapacitors of the two layers will vary in a particular relationship aswill be indicated in accordance with the intensity of the lightimpinging on the device. For example, when the subject device saturatesas the input light level increases, the impedance or equivalentimpedance of the photoconductive layer will decrease absolutely and alsoin relation to the impedance of the electroluminescent layer. Thisdecrease in the impedance of the photoconducting layer causes less ofthe total applied voltage to appear across the photoconductive layer andmore of the applied voltage to be applied across theelectroluminescent-phosphor layer with an attending increase in theoutput brightness. When the impedance of the photoconducting layer hasbecome relatively small compared with the impedance of theelectroluminescent layer no appreciable further change in the voltageacross the electroluminescent layer can occur since most of the appliedvoltage at this point will already be across the electroluminescentlayer. This condition will occur even though the photoconductive layerhas not saturated and even though the electroluminescent layer has notreached its breakdown point. It is this face namely that the voltageacross the electroluminescentphosphor layer can no longer changeappreciably even though the input light intensity should increasesubstantially which is the reason for the occurrence of the outputsaturation that takes place in the subject device. This means of coursethat it is necessary to select an applied voltage that will not causebreakdown of the electroluminescent layer. This is a simple matter.Another way of looking at these same changes that occur in the presentdevice is to consider that as the intensity of the input lightincreases, the impedance characteristics of theelectroluminescent-phosphor and photoconducting layer change in such away that they cause an increase in the proportion of the applied voltageacross the device to be applied across the electroluminescent layer anda decrease in the proportion of the applied voltage to be applied acrossthe photoconducting layer.

In an actual device constructed according to'the present invention theratio of the capacitance of the electroluminescent layer to thecapacitance of the photoconductive layer should be selected to beinitially in a range of from approximately to l to approximately I00 toI. An average value for this ratio will be about 50 to 1. It isnoteworthy in this regard that the capacitance of the photoconductorlayer will decrease with increases in the input light level while thecapacitance of the electroluminescent phosphor layer will decrease withincreased voltage applied across it.

The problem of blooming which is discussed throughout this specificationin large measure is prevented or overcome by also making the layers inthe subject device relatively thin. Also when the subject dynamic rangecompressor is positioned adjacent to an image amplifier device or otheroutput stage, the image amplifier is prevented from blooming because ofthe restricted range of light levels which the dynamic range compressoremits for the reasons stated above. Electrical breakdown of the dynamicrange compressor is also in part prevented or largely overcome by theaddition of the layer of high dielectric constant material which ispositioned between the photoconducting and theelectroluminescentphosphor layers.

Two materials which have been tested for use in the photoconductivelayer are cadmium sulfide (CdS) and cadmium selenide (CdSe). Both ofthese materials show a high degree of photosensitivity in the spectralrange from about 0.7 to 1.0 microns. However, CdS has a lower responsespeed than CdSe. FIG. 4 shows the effective response time of a typicalPC-EL light amplifier as a function of incident illumination. Many otherphotoconductive materials can also be used for some applicationsincluding aluminum arsenide (AlAs) and others. The selection will dependon the desired spectral response, sensitivity, speed of response andother characteristics.

The graph in FIG. 3 shows the output brightness produced by an elementilluminated at relatively low levels of visible light and operating as adynamic range compressor. In the graph both the output brightness andthe incident illumination are in lumens per square meter. The threelines of the graph of FIG. 3 are for different light frequencies usingthe same voltage applied between the conductor layers 26 and 34. Notethe relatively linear relationship that exists between the intensity ofthe incident illumination and the intensity of the output brightnessover the selected range of inputs. In the graph the linear relationshipis shown to exist over a range of more than 2 decades of input lightlevels before saturation begins to take place. This is an importantrelationship because it demonstrates that the subject device is able toreceive andrespond to a relatively wide range of incident lightintensities and this is done in the subject device without producingundesirable blooming of the image. The selection of a photoconductingmaterial is therefore very important to the operation of the subjectdevice and also determines the minimum light level and the light levelrange to which the system is responsive. As already noted the particularphotoconductive material also determines the speed of response, thespectral rangeand other characteristics of the system in which thedevice isused.

The fact that the photoconductive layers continue to respond to incidentlight even when the incident light intensity is sufficiently high tohave caused saturation of output brightness is one of the properties ofthe subject device which is particularly desirable and enables ittoperform as a dynamic range compressor. This is apparent in FIG. 3which shows that no significant increase in the output occurs when theinput increases above the level of about l0 lumens per square meter.Furthermore, no detrimental bloomingtakes place at or near the point ofhighest brightness in the image scene as shown by the graph in FIG. 5which illustrates how the measured profile of the output image of acircular spot of incident light focused into a 5-millimeter diameter onthe output face of an image amplifier varies with changesin theintensity of the incidental lumination of the spot. The particularcurves shown on the graph of FIG. 5 were made from data obtained usingan element such as the element 16 having a uniform layer ofphotoconductive material formed of cadmium selenide (CdSe) and anelectroluminescent-phosphor layer formed of zinc sulfide and copper(ZnSzCu). During the tests no precautions were taken to reduce thetendency for the bright point image impinging on the element to bloom.As seen from the graph, an illumination level of approximately one orderof magnitude greater than anticipated was used with no troublesomedegree of blooming. Furthermore, the blooming can be minimized for evenhigher incident illumination levels to the extent that the pattern ofdeposition of the photoconductor and electroluminescent-phosphor layerscan be utilized without adversely influencing the resolution. Thus, thegraph shown in FIG. 5 clearly illustrates .that by properly selectingthe materials for the photoconductive and electroluminescent-phosphorlayers, dynamic range compression can be obtained which will enable anight sighting device or other similar instrument to be operated over awide range of incident illumination without distorting the image due toundesirable blooming or causing the operator to lose his ability to viewthe target. As already stated, there are many materials that can be usedfor the layers of the element 16 depending on the characteristicsdesired. Care must be taken, however, to select materials which willsatisfy the particular range requirements that are desired and expected.The selected materials will also have an effect on the speed andsensitivity characteristics.

Particular materials which have demonstrated desirable characteristicsfor the photoconductive layer of the subject device include CdS andCdSe. The Cds generally has a lower response speed than CdSe but bothhave similar photosensitivity characteristics. Both of these materialsalso show a high degree of photosensitivity in the spectral range fromabout 0.7 to 1.0 microns which makesthem particularly useful in a nightsight. Relatively few materialsyhowever, possess band gaps in thedesired visible range and at the same time have relatively high mobilitycharacteristics. Aluminum arsenide (AlAs) also appears to be aparticularly good choice for the photoconducting material because it hasa relatively high probability of absorption for incident photons and ahigh mobility for charge carriers generated during this absorptionprocess. The high charge carrier mobility is a necessary characteristicfor a substance to have a fast response time and aluminum arsenide is apromising. material from both of these standpoints. The band gap foraluminum arsenide is also such that its response to near infraredwavelengths is minimized thereby providing means for selectively lookingat a starlight scene without effectively seeing nearinfrared beacons.Other materials can also be used but care must be taken in theirselection. These include in addition to Cds, CdSe and AlAs, substancessuch as lnSb, InAs, HgTe, GhSe, GaAs, GaSb, Ga, lnP, IR, AlP, Si, CdTe,ZnSe, AlSb, ZnS, Gal, SiC, and others.

Various techniques have been developed for constructing continuouslayers of photoconducting material and also continuous layers ofelectroluminescent phosphor. These techniques produce a solid-statelight amplification system which can be operated as a dynamic rangecompressor over as 4 much as 5 or more decades of input brightness withvery little blooming. It is anticipated that even larger dynamic rangescan also be accommodated without serious blooming as the above and othermaterials are explored. It is therefore possible using the teachings ofthe present invention to construct devices capable of responding towidely varying ranges of light intensities without breaking down orbecoming overexcited and blooming. It is also possible using the presentinvention to construct devices capable of sensing extremely minuteamounts of light such as starlight reflected by remote objects. Needlessto say that the principles of the present invention can also be used tosense much larger light intensities, and it is not intended to limit theinvention to use with a night sighting device.

Thus there has been shown and described novel dynamic range compressormeans capable of responding to widely varying light intensities withoutbreaking down or producing undesirable blooming, which means fulfill allof the objects and advantages sought therefor. Many changes,modifications, variations, and other uses and applications of thesubject device will, however, become apparent to those skilled in the anafter considering this specification and the accompanying drawings. Allsuch changes, modifications, variations, and other uses and applicationswhich do not depart from the spirit and scope of the invention aredeemed to be covered by the invention which is limited only by theclaims which follow.

What is claimed is:

1. A dynamic range compressor comprising a semiconductor elementincluding impedance matched layers of photoconductive andelectroluminescent-phosphor materials, a layer of optically opaquedielectric material positioned between the said photoconductive andelectroluminescent-phosphor layers, means establishing a voltage throughthe semiconductor element which is less than the breakdown voltage ofthe electroluminescent-phosphor layer by itself, means exposing thephotoconductive layer to incident light radiations from a remote source,said photoconductive layer being constructed of a material havingrelatively high quantum efficiency and said element having relativelystable output saturation characteristics to produce an output brightnessin the electroluminescent-phosphor layer which varies substantiallylinearly with the intensity of the incident light radiation impingingthereon over a relatively low range of incident light intensities, saidphotoconductive and electroluminescent-phosphor layers being constructedto have an equivalent capacitance relationship when the compressor isnot exposed to incident light in a relationship of from about 1 to toabout 1 to lOO, said relationship changing so that the impedance of theelectroluminescent-phosphor layer increases in relation to the impedanceof the photoconductive layer as the intensity of the incident lightincreases whereby an increasing portion of the voltage establishedthrough the element is across the electroluminescent-phosphor layer, theeffect of the increased voltage across the electroluminescent-phosphorlayer causing the brightness of the image produced thereon to reach amaximum brightness that depends on the voltage applied through theelement thereby limiting the maximum brightness obtained in saidelectroluminescent-phosphor layer and compressing the range ofbrightness of the output for increases in intensity of said incidentlight even in the range of incident light intensities which are highenough to cause substantially all of the applied voltage to be acrossthe electroluminescentphosphor layer, said photoconductive layer beingconstructed of a material capable of responding to incident lightintensity even after the incident light intensity is sufficient to causethe brightness of the image in the electroluminescent-phosphor layer toreach the maximum condition.

2. The dynamic range compressor defined in claim 1 including atransparent electrical conducting layer mounted on opposite surfaces ofthe photoconductive and electroluminescent-phosphor layers formingopposite sides of the element, and a source of voltage connected betweensaid electrical conducting layers, the voltage of said source being lessthan the breakdown voltage of the electroluminescentphosphor layer.

3. The dynamic range compressor defined in claim 1 wherein saidphotoconductive layer is formed of cadmium sulfide.

4. The dynamic range compressor defined in claim 1 wherein saidphotoconductive layer is formed of cadmium selenide.

5. The dynamic range compressor defined in claim 1 wherein saidphotoconductive layer is formed of aluminum arsenide.

6. The dynamic range compressor defined in claim 1 wherein saidelectroluminescent-phosphor layer is formed of a combination of zincsulfide and copper.

7. The dynamic range compressor defined in claim I wherein saidphotoconductive layer is formed of a material having a charge mobilitycharacteristic in square centimeters per volt-second in the range from10 to 10 and a band gap energy ranging from less than 0.5 electron voltsto more than 3 electron volts.

8. A night sight for use under minimum nighttime conditions such asunder starlight comprising a light-gathering mirror positioned to facein the direction of a field of view, said mirror being constructed toconcentrate incident light impinging thereon from the field of view, alight-sensitive element positioned to be exposed to the concentratedincident light, said light-sensitive element positioned to be exposed tothe concentrated incident light, said light-sensitive element includinga multilayer semiconductor wafer element formed of layers of materialsincluding a layer of photoconductive materials for exposing to theincident light, a layer of controlled opacity positioned adjacent to thephotoconductive layer, and an electroluminescent layer on the oppositeside of the opaque layer from the photoconductive layer for reproducingan image of the field of view, means to establishing a predeterminedvoltage across the element that is less then the breakdown voltage ofthe electroluminescent layer by itself, said means including a voltagesource and a pair of light transparent electrically conductive layersattached to opposite sides of the element respectively in contact withthe photoconductive and the electroluminescent layers, saidphotoconductive and electroluminescent layers being impedance matched sothat the equivalent capacitance of the electroluminescent layer to theequivalent capacitance of the photoconductive layer when the waferelement is not exposed to incident light is in a range from about 20 tol to about to l, the electroluminescent layer being constructed of amaterial whose capacitive reactance increases with increases in thevoltage applied thereacross, the photoconductive layer being constructedof a material whose effective impedance decreases with increases in theintensity of the incident light impinging thereon, the relationship ofthe total impedance characteristics of said electroluminescent andphotoconductive layers changing with increases in light intensityreaching a condition at a predetermined incident light intensity wheresubstantially all of the voltage applied across the element is acrossthe electroluminescent layer, said photoconductive layer beingconstructed of a light-sensitive material that continues to respond toincident light intensity even when the incident light intensity issufficient to have caused a substantially maximum upper end of saidrange and does not substantially increase in intensity with furtherincreases in the brightness of the incident light.

10. The night sight defined in claim 8 wherein saidincidentlight-concentrating means includes a concave mirror positionedto face in the direction of the field of view and a convex mirrorpositioned in the path of the concentrated incident light reflected bythe concave mirror to reflect said light onto the photoconductive layerof the semiconductor element FORM 0-1050 l10-69) UNITED STATES PATENTOFFICE CERTIFICATE OF CORRECTION 3,610 ,937 October 5, 1971 Patent No.Dated Inventor(s) James Dueker It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 1, line 6, "device should be "more".

Column 4, line 65, (second occurrence) should be "impedances""impedance" Column 5, line 13, "face" should be "fact".

Column 6, line 21, "10 should be "10' Column 8, line 40, cancel "saidlight-sensitive element positioned to be exposed to the"; line 41,cancel "concentrated incident light,".

Signed and SOLllLZCl this 18th day of April 1972.

(SEAL) Atte st:

EDWARD I LFLETCTT. SH, JR. Attosting Offic 21'' ROBERT GOTTSCHALKCommissioner of Patents USCOMM-DC 6D376-P69 U 5 GOVERNMENT PRINTINGOFFICE 19? O-365-3J4

1. A dynamic range compressor comprising a semiconductor elementincluding impedance matched layers of photoconductive andelectroluminescent-phosphor materials, a layer of optically opaquedielectric material positioned between the said photoconductive andelectroluminescent-phosphor layers, means establishing a voltage throughthe semiconductor element which is less than the breakdown voltage ofthe electroluminescentphosphor layer by itself, means exposing thephotoconductive layer to incident light radiations from a remote source,said photoconductive layer being constructed of a material havingrelatively high quantum efficiency and said element having relativelystable output saturation characteristics to produce an output brightnessin the electroluminescent-phosphor layer which varies substantiallylinearly with the intensity of the incident light radiation impingingthereon over a relatively low range of incident light intensities, saidphotoconductive and electroluminescent-phosphor layers being constructedto have an equivalent capacitance relationship when the compressor isnot exposed to incident light in a relationship of from about 1 to 20 toabout 1 to 100, said relationship changing so that the impedance of theelectroluminescent-phosphor layer increases in relation to the impedanceof the photoconductive layer as the intensity of the incident lightincreases whereby an increasing portion of the voltage establishedthrough the element is across the electroluminescent-phosphor layer, theeffect of the increased voltage across the electroluminescent-phosphorlayer causing the brightness of the image produced thereon to reach amaximum brightness that depends on the voltage applied through theelement thereby limiting the maximum brightness obtained in saidelectroluminescent-phosphor layer and compressing the range ofbrightness of the output for increases in intensity of said incidentlight even in the range of incident light intensities which are highenough to cause substantially all of the applied voltage to be acrossthe electroluminescent-phosphor layer, said photoconductive layer beingconstructed of a material capable of responding to incident lightintensity even after the incident light intensity is sufficient to causethe brightness of the image in the electroluminescent-phosphor layer toreach the maximum condition.
 2. The dynamic range compressor defined inclaim 1 including a transparent electrical conducting layer mounted onopposite surfaces of the photoconductive and electroluminescent-phosphorlayers forming opposite sides of the element, and a source of voltageconnected between said electrical conducting layers, the voltage of saidsource being less than the breakdown voltage of theelectroluminescent-phosphor layer.
 3. The dynamic range compressordefined in claim 1 wherein said photocOnductive layer is formed ofcadmium sulfide.
 4. The dynamic range compressor defined in claim 1wherein said photoconductive layer is formed of cadmium selenide.
 5. Thedynamic range compressor defined in claim 1 wherein said photoconductivelayer is formed of aluminum arsenide.
 6. The dynamic range compressordefined in claim 1 wherein said electroluminescent-phosphor layer isformed of a combination of zinc sulfide and copper.
 7. The dynamic rangecompressor defined in claim 1 wherein said photoconductive layer isformed of a material having a charge mobility characteristic in squarecentimeters per volt-second in the range from 102 to 105 and a band gapenergy ranging from less than 0.5 electron volts to more than 3 electronvolts.
 8. A night sight for use under minimum nighttime conditions suchas under starlight comprising a light-gathering mirror positioned toface in the direction of a field of view, said mirror being constructedto concentrate incident light impinging thereon from the field of view,a light-sensitive element positioned to be exposed to the concentratedincident light, said light-sensitive element positioned to be exposed tothe concentrated incident light, said light-sensitive element includinga multilayer semiconductor wafer element formed of layers of materialsincluding a layer of photoconductive materials for exposing to theincident light, a layer of controlled opacity positioned adjacent to thephotoconductive layer, and an electroluminescent layer on the oppositeside of the opaque layer from the photoconductive layer for reproducingan image of the field of view, means to establishing a predeterminedvoltage across the element that is less then the breakdown voltage ofthe electroluminescent layer by itself, said means including a voltagesource and a pair of light transparent electrically conductive layersattached to opposite sides of the element respectively in contact withthe photoconductive and the electroluminescent layers, saidphotoconductive and electroluminescent layers being impedance matched sothat the equivalent capacitance of the electroluminescent layer to theequivalent capacitance of the photoconductive layer when the waferelement is not exposed to incident light is in a range from about 20 to1 to about 100 to 1, the electroluminescent layer being constructed of amaterial whose capacitive reactance increases with increases in thevoltage applied thereacross, the photoconductive layer being constructedof a material whose effective impedance decreases with increases in theintensity of the incident light impinging thereon, the relationship ofthe total impedance characteristics of said electroluminescent andphotoconductive layers changing with increases in light intensityreaching a condition at a predetermined incident light intensity wheresubstantially all of the voltage applied across the element is acrossthe electroluminescent layer, said photoconductive layer beingconstructed of a light-sensitive material that continues to respond toincident light intensity even when the incident light intensity issufficient to have caused a substantially maximum brightness of theelectroluminescent layer, and means positioned to observe the reproducedimage on the electroluminescent layer.
 9. The night sight defined inclaim 8 wherein said photoconductive layer is constructed of materialhaving a relatively high quantum efficiency and output saturation suchthat the brightness of the image reproduced on the electroluminescentlayer varies substantially linearly with the intensity of the incidentlight impinging on the photoconductive layer over a range of relativelylow incident light intensities, and wherein the output brightness of theelement saturates adjacent to the upper end of said range and does notsubstantially increase in intensity with further increases in thebrightness of the incident light.
 10. The night sight defined in claim 8wherein said incident-light-concentrating means includes a concavemirror positioned to face in the direction of the field of view and aconvex mirror positioned in the path of the concentrated incident lightreflected by the concave mirror to reflect said light onto thephotoconductive layer of the semiconductor element.